Compound Interest Calculator
Project how savings or investments grow with regular contributions, see how much of the final balance is growth versus money you put in, and view the result in today's dollars.
Your details
Optional. Models raises.
Results
Starting with $10,000 and adding $250 a month at 7% compounded monthly, you would have about $170,619 after 20 years: $70,000 contributed and $100,619 in growth. In today's dollars that is about $104,124.
Balance after 20 years
$170,619
about $104,124 in today's dollars
- Contributions41%
- Growth59%
- Total contributed
- $70,000
- Total growth
- $100,619
| Year | Growth that year | Balance | Today's $ |
|---|---|---|---|
| 1 | $821 | $13,821 | $13,484 |
| 2 | $1,097 | $17,918 | $17,055 |
| 3 | $1,393 | $22,312 | $20,719 |
| 4 | $1,711 | $27,023 | $24,481 |
| 5 | $2,052 | $32,074 | $28,349 |
| 6 | $2,417 | $37,491 | $32,329 |
| 7 | $2,808 | $43,300 | $36,427 |
| 8 | $3,228 | $49,528 | $40,650 |
| 9 | $3,679 | $56,207 | $45,006 |
| 10 | $4,161 | $63,368 | $49,503 |
| 15 | $7,152 | $107,730 | $74,384 |
| 20 | $11,392 | $170,619 | $104,124 |
How this calculator works
The classic formula is A = P(1 + r/n)nt: principal P, annual rate r, n compounding periods per year, t years. Because most people also add money over time, this calculator simulates month by month: it converts your rate and compounding choice to an equivalent monthly growth factor, grows the balance, then adds that month's contribution. The "today's dollars" column divides each year's balance by cumulative inflation.
Three levers, in order of importance
- Time. Doubling the years more than doubles the result. Twenty years at 7% turns $250 a month into roughly $130,000; forty years turns it into roughly $650,000.
- Rate. Two percentage points of return compound into a very different outcome over decades, which is why fees matter.
- Contributions. Increasing contributions with raises (the last field) keeps your savings rate steady as income grows.
Frequently asked questions
What is compound interest?
Compound interest is growth on growth: each period, interest is calculated on the original amount plus all the interest already earned. Over long periods this produces a curve that bends upward, which is why starting early matters more than contributing more later.
What rate of return should I assume?
A broad U.S. stock index has returned roughly 10% a year before inflation over long periods, or about 7% after inflation, with large swings year to year. Bonds and savings accounts return less with less volatility. Planners commonly model 6% to 7% for a diversified portfolio and lower for shorter horizons.
Does compounding frequency matter much?
Less than people expect. $10,000 at 5% grows to about $16,289 in 10 years with annual compounding and about $16,470 with monthly compounding. The rate and the time horizon matter far more than the frequency.
Why show results in today's dollars?
Inflation erodes what money buys. A balance of $500,000 in 30 years buys roughly what $240,000 buys today at 2.5% inflation. The "today's dollars" figure divides the future balance by cumulative inflation so you can judge it against current prices.
What is the rule of 72?
A quick mental shortcut: divide 72 by the annual return to estimate how many years it takes money to double. At 6% money doubles in about 12 years; at 9% in about 8.